A storage control unit may have for example a plurality of storage devices such as hard disk drives connected with it. On receiving a write command from a host computer, the storage control unit writes data to at least one storage device of the plurality of storage devices, and, on receiving a read command from the host computer, reads data from at least one storage device of the plurality of storage devices and transmits this data to the host computer.
Such a storage control unit comprises a cache memory for temporarily storing the data that is written to the storage devices in response to a write command or for temporarily storing the data that is read from the storage devices in response to a read command.
As this cache memory, typically volatile memory wherein data can be stored during power is supplied thereto is employed.
In a storage control unit comprising volatile memory as the cache memory if the supply of power to the cache memory ceases due for example to a fault of the external power supply, the data stored in the cache memory is lost.
Accordingly, in order to cope with for example such faults of the external power source, the storage control unit is provided with a battery capable of supplying power, so that, if a fault of the external power source occurs, the data stored in the cache memory can be maintained by supplying power to the cache memory from the battery.
However, since supply of power to the cache memory must be maintained until the fault is cleared, the capacity of the battery must be comparatively large. This leads to the problem that the cost of manufacturing the storage control unit is increased.
In this connection, a technique has been disclosed of preserving data even without using a battery to maintain supply of power of the cache memory until the fault is cleared, by saving the data of the cache memory to non-volatile memory (see for example Laid-open Japanese Patent Application No. 2004-21811).
If for example it is assumed that the data stored in the cache memory is saved in non-volatile memory, if all of the data of the cache memory is to be appropriately saved, it is necessary to provide non-volatile memory having a capacity equivalent to the capacity of the cache memory. This increases the manufacturing cost of the storage control unit.
On the other hand, if, in order to keep down the cost of manufacture, it is arranged to save the data by providing non-volatile memory of smaller capacity than the cache memory, the data of the cache memory cannot be adequately saved to the non-volatile memory, so there is the risk of producing a situation in which necessary data is lost.